Design And Synthesis Of Polyoxometalates/Defective Material-Nitrogen Reduction Photocatalysts And Their Performance Studies

Photocatalytic nitrogen reduction reaction（NRR）,a green and sustainable catalytic conversion pathway of nitrogen,has put forward higher requirements for photocatalysts,such as carrier separation and transport efficiency,nitrogen adsorption and activation performance and light utilization.The defective materials,as an important branch of the nitrogen fixation photocatalyst,can adsorb and activate nitrogen efficiently,reduce the reaction energy barrier and promote the reaction.As a significant branch of the field of nitrogen fixation photocatalysts,defective materials can achieve efficient activation of nitrogen,reduce the energy barrier of the reaction,and promote the occurrence of nitrogen fixation.However,its further development is seriously hindered by low light utilization and carrier separation efficiency.Polyoxometalates（POMs）are a kind of nano-multifunctional metal oxygen clusters,which possess unique physicochemical properties.With the deep integration and convergence of polyoxometalates chemistry,material chemistry and catalytic chemistry,POMs have shown excellent performance in many catalytic fields.The absorption range and carrier separation and transfer of the defective materials are improved by introducing POMs as photosensitizers and electron donors,so as to achieve an efficient and stable photocatalytic nitrogen fixation process.In this thesis,aiming to optimize the kinetic process of photocatalytic nitrogen fixation,a series of photocatalysts with high efficiency and novelty have been designed and applied to the field of nitrogen fixation through screening defective materials,the regulation of POMs composition,and the construction of heterojunctions.The specific work is as follows:1.Based on the wide absorption spectrum of the reduced POMs(r-PMo₁₂,r-PW₁₂,r-P₂Mo₁₈and r-P₂W₁₈),it acted as photosensitizers to modify the defective carbon nitride.Heteropoly blue（r-POMs）nanoclusters as photosensitizers were anchored on the protonated nitrogen vacancy carbon nitride（HV-C₃N₄）,constructed the r-POMs/defective carbon nitride composites（r-POMs/HV-C₃N₄）,which photocatalytic nitrogen reduction performance were studied.The spectral absorption of the composites was enhanced by the introduction of r-POMs nanoclusters.The efficiency of nitrogen fixation on varying degrees,the carrier separation efficiency and interface electron transfer rate can be improved by adjusting the r-POMs type in composites.Among them,r-PW₁₂/HV-C₃N₄ exhibited the best performance(171.4μmol L^-1),which is 94.39%and 86.98%higher than HV-C₃N₄ and r-PW₁₂ respectively.This study has brought new enlightenment for the development of high-efficiency non-precious metal nitrogen reduction photocatalysts.2.The PMoV-based hybrid materials with clear crystal structure and adjustable spectral absorption,as an electronic switch and light capture component,coupled with the defective cocatalyst to establish the“working-in-tandem”mechanism for photocatalytic nitrogen fixation.A series of mixed-addendum PMoV-based hybrid frameworks(abbr.PMo₈V₆,PMo₁₁V_3.5-Ni,PMo₈V₆-Ni)were coupled with sulfur vacancy 1T phase MoS₂（Sv-1T MoS₂）.The intervalence charge transfer（IVCT）of reduced polyoxometalates and the construction of Ni tri-nuclear cluster endow PMo₈V₆-Ni with outstanding light absorption properties.The carrier separation and transfer of PMo₈V₆-Ni were promoted by the Sv-1T MoS₂ cocatalyst.The results showed that the ammonia generation rate of Sv-1T MoS₂/PMo₈V₆-Ni(80.6μmol h^-1 g^-1)is much higher than that of any single component.The sulfur vacancies as nitrogen active sites and polyoxometalate crystalline photosensitizers are extremely crucial for facilitating N₂ chemisorption and NH₃formation.This work provides a fresh perspective for the rational design of photocatalyst with energetic electrons.3.Based on the fact that the reduced polyvanadates have excellent optical properties and reversible multi electron transfer properties,it was used as the photosensitizer and electron donor model to couple with the Cu-doped Zn Al hydrotalcite material（0.5%-Zn Al LDH）with nitrogen fixation activity to prepare a series of heterogeneous catalysts(V₁₀/0.5%-Zn Al,V₁₅/0.5%-Zn Al,V₃₄/0.5%-Zn Al).The results showed that the confined properties of hydrotalcite materials ensure the high dispersion of polyvanadates nanoclusters.The relationship between the reduction degree of vanadium and the nitrogen fixation catalytic activity was studied.With the increase of the reduction degree of vanadium in the polyvanadates(V₁₀^V,V₁₅=V₇^IVV₈^V,V₃₄=V₁₆^IVV₁₈^V),the absorption spectrum of the composites was significantly red shifted,and the carrier separation and transfer efficiency were significantly improved.Among them,V₃₄/0.5%-Zn Al showed the best stability and the nitrogen reduction reaction efficiency(310.4μmol h^-1 g^-1),which is much higher than that of any single component.The reduction state of vanadium has a positive effect on the catalytic activity of the composites.4.Taking advantage of the element composition and adjustable energy level of transition metal substituted polyoxometalates,it was used as an"electronic sponge"model to encapsulate into the ligand-defective metal organic frameworks（d-Ui O-66）to construct host-guest materials（Si W₉X₃@d-Ui O-66,X=W,Fe,V）,and the influence of different transition metals substituted POMs on nitrogen reduction performance was explored.Among them,Si W₉Fe₃@d-Ui O-66 possess the best photocatalytic activity and stability.The nitrogen reduction efficiency of Si W₉Fe₃@d-Ui O-66(98.4μmol h^-1 g^-1)is much higher than that of any single component.The results showed that the difference of transition metal species can improve the electronic structure of the POMs.The introduction of transition metal substituted POMs can broaden the spectral absorption of the composite and inhibit the recombination of carriers.The host-guest materials were constructed to effectively avoid the agglomeration of POMs,and the synergistic effect between the material components optimized the nitrogen fixation activity.